Drop on demand ink jet technology is widely used in the printing industry. Printers using drop on demand ink jet technology can use either thermal ink jet technology or piezoelectric technology. Even though they are more expensive to manufacture than thermal ink jets, piezoelectric ink jets are generally favored, for example because they can use a wider variety of inks.
In solid inkjet printing technology, ink, in solid form, is heated to a printing temperature and ejected from a printhead nozzle by a plurality of ejectors (actuators). The ink can be deposited, for example, directly onto a print medium or onto a media transfer device such as a heated rotating drum which transfers the ink through physical contact with the print medium.
To provide suitable print quality using solid inkjet printers, it is desirable to dispense ink from the ejectors at a temperature which is within a few degrees of a target temperature. The target temperature for solid ink can be between about 105° C. and 140° C. The temperature of the melted ink can be maintained by heating the printhead with a heated mass such as a flexible polyimide thin film layer with metal traces of gold or copper on the polyimide surface. The heater is assembled using adhesive layers, and heats the printhead which transfers the heat through contact with the ink as it flows through channels in the printhead.
Piezoelectric ink jet print heads include an array of piezoelectric elements (i.e., transducers or PZTs), where the array includes an interspatial gap between adjacent piezoelectric elements to provide the correct spacing between each piezoelectric element. Piezoelectric ink jet print heads can typically further include a flexible diaphragm to which the array of piezoelectric elements is attached. When a voltage is applied to a piezoelectric element, typically through electrical connection with an electrode electrically coupled to a power source, the piezoelectric element bends or deflects, causing the diaphragm to flex which expels a quantity of ink from a chamber through a nozzle. The flexing further draws ink into the chamber from a main ink reservoir through an opening to replace the expelled ink.
To attach an array of piezoelectric elements to pads or electrodes of a flexible printed circuit (flex circuit) or to a printed circuit board (PCB), a quantity (e.g., a microdrop) of conductor such as conductive epoxy, conductive paste, or another conductive material is dispensed individually on the top of each piezoelectric element. Electrodes of the flex circuit or PCB are placed in contact with each microdrop to facilitate electrical communication between each piezoelectric element and the electrodes of the flex circuit or PCB.
Achieving reliable electrical connections or interconnects between piezoelectric elements and a circuit layer becomes more challenging at increasing print head resolutions. Design constraints that require dimensionally smaller PZTs reduce both the surface area available for forming an electrical interconnect as well as the area for its surrounding bond adhesive, which can result in weaker electrical interconnects that may fail after stressing due, for example, to thermal cycling, thermal aging, and PZT actuations.
To operate a piezoelectric printhead, electrodes should be in contact with individual lead zirconate titanate (PZT) elements. In piezoelectric printhead with 600 DPI or 1200 DPI, either silver epoxy or bumped flex circuit is used to address individual PZT elements. As the resolution of the printhead increases, the area of individual PZT elements decreases. Therefore, the alignment of silver epoxy and bumped flex circuits to PZT elements will be more difficult. On the other hand, because the contact areas between silver epoxy or bumped flex circuits and individual PZT elements are large, the deflection of PZT is hindered.
What is needed, then, are improved high-density three-dimensional interconnection approach for piezoelectric printheads.